Big Bang

Time line of the Universe. The expansion of the universe
over most of its history has been relatively gradual. The notion that
a rapid period of "inflation" preceded the Big Bang expansion was
first put forth 25 years ago. Recent observations, including those
by NASA's WMAP orbiting ob era tory favor specific inflation scenarios
over other long held ideas. Credit: NASA.

unification of forces in the early universe.

The Big Bang is the event in which, according to standard modern cosmology, the Universe
came into existence some 12 to 15 billion years ago. It is sometimes
described as an "explosion;" however, it is wrong to suppose that matter
and energy erupted into a pre-existing space. Modern Big Bang theory holds
that space and time came into being simultaneously with matter and energy. The possible overall
forms that space and time could take – closed, open, or flat –
are described by three different cosmological
models.

Creation to inflation

According to current theory, the first physically distinct period in the
Universe lasted from "time zero" (the Big Bang itself) to 10-43 second later, when the universe was about 100 million trillion times smaller
than a proton and had a temperature of 1034 K. During this so-called Planck era, quantum gravitational effects dominated
and there was no distinction between (what would later be) the four fundamental
forces of nature – gravity, electromagnetism,
the strong force, and the weak
force. Gravity was the first to split away, at the end of the Planck
era, which marks the earliest point at which present science has any real
understanding. Physicists have successfully developed a theory that unifies
the strong, weak, and electromagnetic forces, called the Grand Unified Theory
(GUT). The GUT era lasted until about 10-38 second after the
Big Bang, at which point the strong force broke away from the others, releasing,
in the process, a vast amount of energy that, it is believed, caused the
Universe to expand at an extraordinary rate. In the brief ensuing interval
of so-called inflation, the Universe
grew by a factor of 1035 (100 billion trillion trillion) in 10-32 seconds, from being unimaginably smaller than a subatomic particle to about
the size of a grapefruit.

Postulating this burst of exponential growth helps remove two major problems
in cosmology: the horizon problem and the flatness problem. The horizon
problem is to explain how the cosmic
microwave background – a kind of residual glow of the Big Bang
from all parts of the sky – is very nearly isotropic despite the fact that the observable universe isn't yet old enough for light,
or any other kind of signal, to have traveled from one side of it to the
other. The flatness problem is to explain why space, on a cosmic scale,
seems to be almost exactly flat, leaving the universe effectively teetering
on a knife-edge between eternal expansion and eventual collapse. Both near-isotropy
and near-flatness follow directly from the inflationary scenario.

Electroweak era (10-38 to 10-10 second)

At the end of inflationary epoch, the so-called vacuum energy of space underwent
a phase transition (similar to when water vapor in the atmosphere condenses
as water droplets in a cloud) suddenly giving rise to a seething soup of
elementary particles, including photons, gluons, and quarks.
At the same time, the expansion of the universe dramatically slowed to the
"normal" rate governed by the Hubble law. At about 10-10 seconds,
the electroweak force separated into the electromagnetic and weak forces,
establishing a universe in which the physical laws and the four distinct
forces of nature were as we now experience them.

Particle
era (10-10 to 1 second)

The biggest chunks of matter, as the Universe ended its first trillionth
of a second or so, were individual quarks and their antiparticles,
antiquarks – the underlying particles out of which future atoms, asteroids,
and astronomers would be made. As time went on, quarks and anti-quarks annihilated
each other. However, either because of a slight asymmetry in the behavior
of the particles or a slight initial excess of particles over antiparticles,
the mutual destruction ended with a surplus of quarks. Only because of this
(relatively minor) discrepancy do stars, planets, and human beings exist
today.

Between 10-6 and 10-5 second after the beginning of
the Universe, when the ambient cosmic temperature had fallen to a balmy
1015 K, quarks began to combine to form a variety of hadrons.
All of the short-lived hadrons quickly decayed leaving only the familiar
protons and neutrons of which the nuclei of atoms-to-come would be made.
This hadron era was followed by the lepton era, during which most of the matter in the Universe consisted of leptons
and their antiparticles. The lepton era drew to a close when the majority
of leptons and antileptons annihilated one another, leaving, again, a comparatively
small surplus to populate the future universe.

One to
100 seconds

Up to this stage, neutrons and protons had been rapidly changing into each
other through the emission and absorption of neutrinos.
But, by the age of one second, the Universe was cool enough for neutron-proton
transformations to slow dramatically. A ratio of about seven protons for
every neutron ensued. Since to make a hydrogen nucleus, only one proton is needed, whereas helium requires two protons
and two neutrons, a 7:1 excess of protons over neutrons would lead to a
similar excess of hydrogen over helium –
which is what is observed today. At about the 100-second mark, with the
temperature at a mere billion K, neutrons and protons were able to stick
together. The majority of neutrons in the Universe wound up in combinations
of two protons and two neutrons as helium nuclei. A small proportion of
neutrons contributed to making lithium, with three protons and three neutrons,
and the leftovers ended up in deuterium – an isotope of hydrogen with one proton and one neutron.

The first 10,000 years

Most of the action, at the level of particle physics, was compressed into
the first couple of minutes after the Big Bang. Thereafter, the universe
settled down to a much lengthier period of cooling and expansion in which
change was less frenetic. Gradually, more and more matter was created from
the high energy radiation that bathed the cosmos. The expansion of the Universe,
in other words, caused matter to lose less energy than did the radiation,
so that an increasing proportion of the cosmic energy density came to be
invested in nuclei rather than in massless, or nearly massless, particles
(mainly photons). From a situation in which the energy invested in radiation
dominated the expansion of spacetime, the Universe evolved to the point
at which matter became the determining factor. Around 10,000 years after
the Big Bang, the radiation era drew to a close and the matter era began.

When the Universe became transparent

About 300,000 years after the Big Bang, when the cosmic temperature had
dropped to just 3,000 K, the first atoms formed. It was then cool enough
to allow protons to capture one electron each and form neutral atoms of
hydrogen. While free, the electrons had interacted strongly with light and
other forms of electromagnetic radiation, making the Universe effectively
opaque. But bound up inside atoms, the electrons lost this capacity, matter
and energy became decoupled, and, for the first time, light could travel
freely across space. This, then, marks the earliest point in time to which
we can see back. The cosmic microwave background is the greatly redshifted
first burst of light to reach us from the early Universe and provides an
imprint of what the Universe looked like about a third of a million years
after the Big Bang. Fluctuations in the nearly-uniform density of the infant
Universe show up as tiny temperature differences in the microwave background
from point to point in the sky. These fluctuations are believed to be the
seeds from which future galaxies and clusters of galaxies arose.